Compared to other printing methods, the inkjet recording method offers ease of full-color printing since it involves a simpler process. And also, the inkjet recording method has the benefit of enabling the production of high-resolution images despite being of simple constitution.
As inkjet recording inks, dye-based inks are used in which various types of water-soluble dyes are dissolved in water or a mixture of water and an organic solvent. Such dye-based inks have a disadvantage of having poor light resistance, though they are excellent in color tone sharpness. Meanwhile, pigment-based inks in which carbon black and various types of organic pigments are dispersed are superior in light resistance compared to dye-based inks, and thus studies on pigment-based inks are being intensively made.
However, pigment-based inks are more likely to cause nozzle clogging compared to dye-based inks. Pigment inks are generally prepared as follows. A coloring material and a dispersant are preliminarily dispersed in an aqueous solvent to prepare a dispersion product. After that, the dispersion product is dispersed to a predetermined degree by a media-type disperser, followed by diluting to a predetermined concentration.
Water-based pigment inks usually contain a surfactant and a water-soluble resin to disperse a hydrophobic pigment therein, but the reliability of images obtained using such inks is extremely low. Thus, according to some disclosed techniques, film-formable resin fine particles are added into the inks in aim of improving the quality of images. However, it is difficult to keep a plurality of components in a state of being finely and stably dispersed throughout the ink for a long period of time. And so, when a dispersant such as a surfactant is used in a large amount in order to stably disperse such fine particles, a problem inconveniently occurs. Specifically, the problem, for example, is that air bubbles may be generated inside the ink tank and/or inkjet head, which could cause degradation in image quality. Likewise, in aim of improving dispersibility, research has been conducted into the effectiveness of methods that change the surface of a pigment to a hydrophilic group, or, the effectiveness of employing a resin containing a hydrophilic group. However, while a given component may be stable when used alone, combined use of several different components has problematically caused degradation of dispersion stability as well as degradation of storage stability.
In addition, there have been reports of a surface treated pigment ink excelling in storage stability and jetting stability (PTLs 1 to 3); an ink set able to maintain a stable inkjet (PTL 4); a method of incorporating, into an ink, a water-insoluble color material and charged resin pseudo-fine particles smaller than the color material in order to attain a high image density (PTL 5); a method of incorporating a self-dispersible pigment which is defined in term of a DBP oil absorption amount (PTL 6); and an aqueous carbon black dispersion liquid containing a surface reformed carbon black and a nonionic surfactant that has an HLB value of 7 to 18 and has an acetylene skeleton (PTL 7). There is also report of an aqueous ink employing an acetylene glycol surfactant (PTL 8).
Methods for the purpose of stabilizing dispersion have been reported. Examples of these methods include a method of dispersing in water a water-dispersible resin having a carboxyl group and a nonionic hydrophilic group in its structure (PTL 9); a method of either unifying the polarities of a water-soluble polymer and a surfactant or adding nonions (PTL 10); a method of unifying the polarities of colored ion-containing polyester resin and a hydrophilic group of a colorant (PTL 11) and a method of unifying the dispersion polarities of pigment and resin fine particles (PTL 12). There is also disclosure of a printing ink employing gemini-type surfactant as a dispersant (PTL 13).
Also, there have been reports of an aqueous inkjet recording ink composition containing a pigment dispersion liquid in which at least 70% of the particles in the dispersion liquid have a diameter of less than 0.1 μm, and the other particles in the dispersion liquid have a diameter equal to or less than 0.1 μm, an aldehyde naphthalene sulfonate dispersant and/or at least one sulfone solvent (PTL 14).
In PTL 15, there is proposed a recording liquid consisting of an aqueous medium containing a pigment, a polymer dispersant and a nonionic surfactant. In PTLs 16 and 17, there is also proposed use of an AB or BAB block copolymer as a pigment dispersant. And, in PTL 18, there is proposed use of a specific pigment, a water-soluble resin and a solvent.
As a pigment dispersion method which does not use a dispersant, several methods have been disclosed. Examples of such methods include a method of introducing, into carbon black, a substituent containing a water-soluble group (PTL 19); a method of polymerizing a water-soluble monomer or the like on the surface of carbon black (PTL 20); and a method of oxidizing carbon black (PTL 21). Also, there is disclosed a method of attaining water resistance and jetting stability by incorporating, into an ink, an oxidized carbon black and a terpolymer of acrylic acid, styrene and α-methyl styrene (PTL 22).
There is also proposed an inkjet recording liquid wherein the volume average particle diameter of the dispersion particles in the inkjet recording liquid is in the range of 30 nm to 200 nm (PTL 23).
However, while ink liquids of the aforementioned conventional methods could attain high image density in the case of color pigment inks, they were not satisfactory in that they could not provide sufficient performance in the case of black color pigment inks. Also, examples of using beads of a diameter in the range of about 0.05 mm to about 1.0 mm in bead mill dispersion have been filed (PTLs 24 to 26), they do not exhibit satisfactory dispersion stability.
Also, use of an anionic surfactant as the dispersant is described in PTL 27. Although the anionic surfactant preferably has a molecular weight (m) falling within the range of 1,000≦m≦30,000 according to the description, the anionic surfactant is considered to be unsatisfactory as far as dispersion stability is concerned. Some pigments that are vulnerable to strong shock during dispersion lack in stability after dispersion, and cause severe problems regarding jetting stability of the formed ink liquid.